Efficient integration of manufacturing of upcycled concrete product into power plants

ABSTRACT

A manufacturing process of a concrete product includes: (1) extracting calcium from solids as portlandite; (2) forming a cementitious slurry including the portlandite; (3) shaping the cementitious slurry into a structural component; and (4) exposing the structural component to carbon dioxide sourced from a flue gas stream, thereby forming the concrete product.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International ApplicationNo. PCT/US2017/058359, now WO 2018/081310, filed Oct. 25, 2017, whichclaims the benefit of U.S. Provisional Application No. 62/413,365, filedOct. 26, 2016, the contents of which are incorporated herein byreference in their entirety. This application also claims the benefit ofU.S. Provisional Application No. 62/566,091, filed Sep. 29, 2017, thecontents of which are incorporated herein by reference in theirentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under grant number1253269, awarded by the National Science Foundation. The Government hascertain rights in the invention.

TECHNICAL FIELD

This disclosure generally relates to manufacturing processes of concreteproducts and systems for manufacturing concrete products.

BACKGROUND

Electricity generation from coal-fired power plants represents about 25%of total carbon dioxide (CO₂) emissions from the United States (about1.4 billion tons of CO₂ emitted in 2015). In view of regulations thatseek to restrict CO₂ emissions in support of climate change goals, it isexpected that such emissions will be financially penalized. The expectedrestriction is of great consequence to emission intensive sectors suchas coal-fired power generation, which are expected to be substantiallyburdened by such penalties.

Carbon capture and storage (CCS) has been proposed as a solution tomitigate anthropogenic CO₂ emissions. However, CCS is not always aviable solution, for: (i) reasons of cost which is estimated to rangefrom about $60-to-$150 (in terms of US dollars) per ton of CO₂, (ii) thepermanence (or lack thereof) of a sequestration solution, and/or (iii)the lack of suitable geological features in a local vicinity where CCScan be favorably achieved. This is further complicated by increasinglevels of anthropogenic CO₂ emissions which render current proposals ofCCS a short-term solution.

Technologies have been proposed to produce concrete products byutilizing CO₂ to carbonate portlandite or wollastonite. However, theproposed technologies specify the use of newly mined or producedmaterials as precursors, and involve energy-intensive processing and,hence, high costs, which can impede the propagation of such technologiesas a viable solution to mitigate anthropogenic CO₂ emissions. Moreover,carbon capture and utilization (CCU) based on mineralization of carbondioxide (CO₂) (carbonation) relies on achieving a high reaction speed(thus a high production throughput) with a low energy input. However, ina typical carbonation reaction, CO₂ is consumed and removed from a gasmixture continuously during the reaction. Therefore in a closed system,the CO₂ concentration and its partial pressure reduce as the reactionproceeds. This substantially impacts the late stage reaction speed andrestricts a final carbonation extent.

It is against this background that a need arose to develop theembodiments described herein.

SUMMARY

In some embodiments, a manufacturing process of a concrete productincludes: (1) extracting calcium from solids as portlandite; (2) forminga cementitious slurry including the portlandite; (3) shaping thecementitious slurry into a structural component; and (4) exposing thestructural component to carbon dioxide sourced from a flue gas stream,thereby forming the concrete product.

In some embodiments of the manufacturing process, the solids include atleast one of iron slag or steel slag.

In some embodiments of the manufacturing process, extracting the calciumincludes subjecting the solids to dissolution in a reactor to yield anion solution, and wherein the leaching reactor is operated using heatsourced from the flue gas stream.

In some embodiments of the manufacturing process, extracting the calciumfurther includes inducing precipitation of the ion solution in aprecipitation reactor to yield the portlandite, and wherein theprecipitation reactor is operated using heat sourced from the flue gasstream.

In some embodiments of the manufacturing process, forming thecementitious slurry includes combining fly ash, bottom ash, economizerash including so called off-spec ashes, or other combustion residualswith the portlandite.

In some embodiments of the manufacturing process, shaping thecementitious slurry includes casting, extruding, molding, pressing, or3D printing of the cementitious slurry.

In some embodiments of the manufacturing process, exposing thestructural component includes exposing, during an initial time period,the structural component to a first gas reactant having a first carbondioxide concentration, followed by exposing, during a subsequent timeperiod, the structural component to a second gas reactant having asecond carbon dioxide concentration that is greater than the firstcarbon dioxide concentration.

In additional embodiments, a system for manufacturing a concrete productincludes: (1) a leaching reactor; (2) a precipitation reactor connectedto the leaching reactor; and (3) a set of heat exchangers thermallyconnected to the leaching reactor and the precipitation reactor andconfigured to source heat from a flue gas stream.

In some embodiments of the system, the set of heat exchangers includes aset of finned-tube heat exchangers.

In some embodiments of the system, the system further includes acapacitive concentrator connected between the leaching reactor and theprecipitation reactor. In some embodiments, the capacitive concentratorincludes a set of electrodes and an electrical source connected to theset of electrodes.

In some embodiments of the system, the system further includes acarbonation reactor connected to the leaching reactor and theprecipitation reactor and configured to source carbon dioxide from theflue gas stream.

In some embodiments of the system, the system further includes a mixerconnected between the leaching reactor, the precipitation reactor, andthe carbonation reactor.

In some embodiments of the system, the system further includes anextruder or a pressing, molding, or forming device connected between themixer and the carbonation reactor.

In some embodiments of the system, the carbonation reactor includes: (i)a reaction chamber; and (ii) a gas exchange mechanism connected to thereaction chamber and configured to: expose, during an initial timeperiod, contents of the reaction chamber to a first gas reactant havinga first carbon dioxide concentration; and expose, during a subsequenttime period, the contents to a second gas reactant having a secondcarbon dioxide concentration that is greater than the first carbondioxide concentration.

In additional embodiments, a manufacturing process of a concrete productincludes: (1) forming a cementitious slurry including fly ash; (2)shaping the cementitious slurry into a structural component; and (3)exposing the structural component to carbon dioxide sourced from a fluegas stream, thereby forming the concrete product.

In some embodiments of the manufacturing process, forming thecementitious slurry includes combining water with the fly ash.

In some embodiments of the manufacturing process, the fly ash includescalcium in the form of one or more calcium-bearing compounds (e.g., lime(CaO)) in an amount of at least about 15% by weight, at least about 18%by weight, at least about 20% by weight, at least about 23% by weight,or at least about 25% by weight, and up to about 27% by weight, up toabout 28% by weight, or more, along with silica (SiO₂) and oxides ofmetals.

In some embodiments of the manufacturing process, shaping thecementitious slurry includes casting, extruding, molding, pressing, or3D printing of the cementitious slurry.

In some embodiments of the manufacturing process, the flue gas streamhas a carbon dioxide concentration equal to or greater than about 3%(v/v).

In some embodiments of the manufacturing process, exposing thestructural component includes exposing, during an initial time period,the structural component to a first gas reactant having a first carbondioxide concentration, followed by exposing, during a subsequent timeperiod, the structural component to a second gas reactant having asecond carbon dioxide concentration that is greater than the firstcarbon dioxide concentration.

In additional embodiments, a manufacturing process includes: (1)introducing, during a first stage, a first gas reagent including carbondioxide to react with a carbon dioxide-capturing reagent, followed by(2) introducing, during a second stage, a second gas reagent includingcarbon dioxide to react with the carbon dioxide-capturing reagent.

In some embodiments of the manufacturing process, a concentration ofcarbon dioxide in the second gas reagent, as introduced, is greater thana remaining concentration of carbon dioxide in the first gas reagentupon completion of the first stage.

In some embodiments of the manufacturing process, a concentration ofcarbon dioxide in the second gas reagent, as introduced, is greaterthan, or substantially the same as, a concentration of carbon dioxide inthe first gas reagent, as introduced. In some embodiments, theconcentration of carbon dioxide in the second gas reagent, asintroduced, is at least about 1.1 times greater than, at least about 1.3times greater than, at least about 1.5 times greater than, at leastabout 2 times greater than, at least about 2.5 times greater than, or atleast about 3 times greater than the concentration of carbon dioxide inthe first gas reagent, as introduced.

In some embodiments of the manufacturing process, introducing the secondgas reagent includes replacing the first gas reagent with the second gasreagent.

In some embodiments of the manufacturing process, the carbondioxide-capturing reagent includes at least one of portlandite orbrucite (Mg(OH)₂).

In some embodiments of the manufacturing process, a partial pressure ofcarbon dioxide in the second gas reagent, as introduced, is in a rangeof about 0.004 MPa to about 0.02 MPa, about 0.1 MPa to about 2 MPa,about 0.5 MPa to about 1.5 MPa, about 0.8 MPa to about 1.2 MPa, or about1 MPa. In some embodiments, the partial pressure of carbon dioxide inthe second gas reagent, as introduced, is greater than a remainingpartial pressure of carbon dioxide in the first gas reagent uponcompletion of the first stage. In some embodiments, the partial pressureof carbon dioxide in the second gas reagent, as introduced, is greaterthan, or substantially the same as, a partial pressure of carbon dioxidein the first gas reagent, as introduced. In some embodiments, thepartial pressure of carbon dioxide in the second gas reagent, asintroduced, is at least about 1.1 times greater than, at least about 1.3times greater than, at least about 1.5 times greater than, at leastabout 2 times greater than, at least about 2.5 times greater than, or atleast about 3 times greater than the partial pressure of carbon dioxidein the first gas reagent, as introduced. In some embodiments, thepartial pressure of carbon dioxide in the first gas reagent changes(e.g., decreases) over the duration of the first stage, relative to thepartial pressure of carbon dioxide in the first gas reagent, asintroduced. In some embodiments, the partial pressure of carbon dioxidein the second gas reagent changes (e.g., decreases) over the duration ofthe second stage, relative to the partial pressure of carbon dioxide inthe second gas reagent, as introduced.

In further embodiments, a carbonation reactor includes: (1) a reactionchamber; and (2) a gas exchange mechanism connected to the reactionchamber, wherein the gas exchange mechanism is configured to: introduce,during a first stage, a first gas reagent including carbon dioxide intothe reaction chamber to react with contents of the reaction chamber; andintroduce, during a second stage, a second gas reagent including carbondioxide into the reaction chamber to react with the contents of thereaction chamber. In some embodiments, a partial pressure of carbondioxide in the first gas reagent changes (e.g., decreases) over theduration of the first stage, relative to the partial pressure of carbondioxide in the first gas reagent, as introduced. In some embodiments, apartial pressure of carbon dioxide in the second gas reagent changes(e.g., decreases) over the duration of the second stage, relative to thepartial pressure of carbon dioxide in the second gas reagent, asintroduced.

Other aspects and embodiments of this disclosure are also contemplated.The foregoing summary and the following detailed description are notmeant to restrict this disclosure to any particular embodiment but aremerely meant to describe some embodiments of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of some embodimentsof this disclosure, reference should be made to the following detaileddescription taken in conjunction with the accompanying drawing.

FIG. 1. An illustration of a manufacturing process flow and itsintegration into a primary exhaust stream of a coal-fired power plant.

FIG. 2. An illustration of capacitive concentration.

FIG. 3. An illustration of the integration of a process flow to tap aflue gas stream prior to and after desulfurization to secure waste heat,and to provide CO₂ for upcycled concrete production.

FIG. 4A. Extent of carbonation of Ca(OH)₂ particulates as a function oftime at different CO₂ pressures.

FIG. 4B. Data of the extent of carbonation after about 1 hour as afunction of CO₂ partial pressure.

FIG. 4C. An illustration of a two-stage carbonation process. Conditionsduring an example setup for gas-fired flue gas stream are indicated.

FIG. 5. A schematic of a carbonation reactor showing vapor streams,sample placement, and monitoring and control units (e.g., flow-meters,pressure regulators, temperature/relative humidity (T/RH) meters, andgas chromatograph (GC)).

FIG. 6. The evolution of compressive strengths of: (a) Ca-rich andCa-poor fly ash pastes following CO₂ exposure at about 75° C., and thecontrol samples (exposed to pure N₂) for comparison, as a function of(carbonation) time in FIG. 6A, (b) hydrated OPC pastes at different agesafter curing in limewater at about 23° C., as a function of w/s in FIG.6B. The dashed black line shows the compressive strength of a Ca-richfly ash formulation following its exposure to CO₂ at about 75° C. forabout 7 days, (c) Ca-rich fly ash pastes carbonated at differenttemperatures following exposure to about 99.5% CO₂ (v/v) and simulatedflue gas (about 12% CO₂, v/v), as a function of time in FIG. 6C, and,(d) Ca-enriched (with added Ca(OH)₂, or dissolved Ca(NO₃)₂) Ca-poor(Class F) fly ash pastes following CO₂ exposure at about 75° C., as afunction of time in FIG. 6D. The compressive strengths of the pristineCa-poor fly ash with and without carbonation are also shown forcomparison.

FIG. 7. GEMS-calculated solid phase balances as a function of the extentof fly ash reaction for Ca-rich and Ca-poor fly ash in the presence of agas-phase composed of: (a, d) air in FIG. 7A and FIG. 7D, (b, e) about12% CO₂ (simulated flue gas environment) in FIG. 7B and FIG. 7E, and (c,f) about 100% CO₂ at T=75° C. and p=1 bar for w/s=0.20 in FIG. 7C andFIG. 7F. Here, ½FH₃═Fe(OH)₃, ½AH₃═Al(OH)₃, and C—S—H=calcium silicatehydrate. The solid phase balance is calculated until the pore solutionis exhausted, or the fly ash reactant is completely consumed.

FIG. 8. Representative X-ray diffraction (XRD) patterns of Ca-rich andCa-poor fly ash formulations before and after exposure to CO₂ at about75° C. for about 10 days. The Ca-poor fly ash shows no noticeable changein the nature of compounds present following exposure to CO₂.

FIG. 9. Representative scanning electron microscopy (SEM) micrographsof: (a) a Ca-rich fly ash formulation following exposure to N₂ at about75° C. for about 10 days in FIG. 9A; a magnified image highlighting thesurface of a fly ash particle is shown in FIG. 9B, (c) a Ca-rich fly ashformulation following exposure to pure CO₂ at about 75° C. for about 10days in FIG. 9C; a magnified image highlighting the surface of acarbonated fly ash particle wherein carbonation products in the form ofcalcite are visible on the particle surface is shown in FIG. 9D, (e) aCa-poor fly ash formulation following exposure to pure CO₂ at about 75°C. for about 10 days in FIG. 9E, and (f) Ca(OH)₂-enriched Ca-poor flyash formulation following exposure to pure CO₂ at about 75° C. for about10 days in FIG. 9F, wherein the somewhat increased formation of calciteis noted on particle surfaces.

FIG. 10. (a) The CO₂ uptake (normalized by the mass of Ca-rich fly ashin the formulation) as a function of time for samples exposed to pureCO₂ at different isothermal temperatures in FIG. 10A. The amount of CO₂uptake was estimated using the mass-based method. (b) The compressivestrength of the Ca-rich and Ca-poor fly ash samples as a function oftheir CO₂ uptake following exposure to pure CO₂ at differenttemperatures for up to about 10 days in FIG. 10B. The data reveals astrength gain rate of about 3.2 MPa per unit mass of fly ash that hasreacted (carbonated). The amount of CO₂ uptake was estimated using themass-based method. (c) The CO₂ uptake of a Ca-rich fly ash formulationas a function of depth in FIG. 10C. The macroscopic sample is composedof a cube (about 50 mm×about 50 mm×about 50 mm) that was exposed to pureCO₂ at about 75° C. for about 10 days. Herein, CO₂ uptake was assessedby thermal analysis (TGA).

FIG. 11. Fits of an equation for a generalized reaction-diffusion modelto experimental carbonation data taken from FIG. 7A for differentcarbonation temperatures.

DETAILED DESCRIPTION

Embodiments of this disclosure are directed to an upcycled concreteproduct. In some embodiments, the use of limestone as a cementationagent is leveraged to result in a CO₂-negative concrete product. Theupcycled concrete product leverages a process to secure calcium speciesfor carbonate mineralization using industrial wastes as precursors orreactants, thereby eliminating the need for newly mined or producedmaterials. Also, a carbonation process can efficiently utilize both CO₂and waste heat carried by flue gas in a fossil fuel power plant, acement plant, and a petrochemical facility, amongst others. In suchmanner, the upcycled concrete product and process can significantlyenhance a CO₂ capturing capacity of a limestone-cement-based concreteproduct, and thereby can establish a CO₂-negative process that canmitigate CO₂ emission at large scales.

An upcycled concrete product is a transformative, CO₂-negativeconstruction material which provides a solution for CO₂ and industrialwaste upcycling. In some embodiments, a manufacturing process of theupcycled concrete product is designed to integrate as a bolt-on systemto coal-fired power plants. Therefore, provision is made to secure fluegas, before desulfurization (or other air pollution control operations),as a heat transfer fluid, and post-desulfurization as a source of CO₂(e.g., equal to or greater than about 3% CO₂ or about 12% CO₂, v/v).Thus, heat provisioned by the flue gas is used to facilitate leachingand precipitation reactions (e.g., above about 20° C., above about 25°C., or above about 35° C.), and accelerate the carbonation kinetics(e.g., above about 20° C., above about 25° C., or above about 35° C.).Furthermore, the CO₂ present in the flue gas is systematically consumedby mineralization. By tapping the flue gas stream at two discretepoints, extrinsic energy demands for upcycled concrete processing arereduced, without imposing additional demands for emissions control.

A manufacturing process flow of some embodiments is illustrated inFIG. 1. The initial stages involve leaching and precipitation ofportlandite (Ca(OH)₂) particulates from reclaimed solids. For example,the reclaimed solids can be in the form of either, or both, crystallizediron slags or steel slags rich in calcium (Ca) and magnesium (Mg). Forexample, the slags can be formed as by-products of iron and steelmanufacturing, and can include calcium in the form of simple oxides(e.g., lime (CaO)) in an amount of at least about 25% by weight, atleast about 30% by weight, at least about 35% by weight, or at leastabout 40% by weight, and up to about 45% by weight, up to about 50% byweight, or more, along with silica (SiO₂) and oxides of metals, such asmagnesia, alumina, manganese oxide, and iron oxide. The slags can besuitably granulated in the form of granules to facilitate subsequentprocessing, such as through greater surface area and associatedinterface effects. The calcium present in the slags is leached orextracted by dissolution in, or exposure to, a leaching solution (e.g.,an aqueous solution optionally including one or more leaching aids) toform a calcium ion solution in a leaching reactor 102 (e.g., a leachingtank) operated at a temperature in a range of about 20° C. to about 90°C. Then, following a controlled concentration of the calcium (in theform of calcium ions) in the leaching solution in a capacitiveconcentrator 104 connected to the leaching reactor 102 and operated at atemperature in a range of about 20° C. to about 25° C., a resultingconcentrated ion solution is induced to precipitate portlandite to yielda portlandite slurry in a precipitation reactor 106 (e.g., aprecipitation tank) connected to the capacitive concentrator 104 andoperated at a temperature in a range of about 70° C. to about 90° C. Insome embodiments and referring to FIG. 2, capacitive concentration isperformed by applying an electrical input from an electrical source 202to a pair of electrodes 204 and 206 included in the capacitiveconcentrator 104, such that calcium ions in the leaching solution aredrawn towards one of the electrodes 204 and 206, and subsequently can bereleased by reversing the electrical input to yield a higherconcentration of the calcium ions. Concentration of the calcium ionsalso can be performed through membrane filtration, such as using ananofiltration membrane or a reverse osmosis membrane.

Referring to FIG. 1, the portlandite slurry and leached slag granulesare then combined with water, fly ash (or other coal combustionby-products), and fine and coarse aggregates using a mixer 108 to form acementitious slurry (e.g., either a concrete or mortar concrete slurry),which is then shape-stabilized into structural components by an extruder110 connected to the mixer 108. Examples of suitable aggregates includesand, gravel, crushed stone, slag, recycled concrete, and so forth.Shape stabilization can yield the structural components as beams,columns, slabs, wall panels, cinder blocks, bricks, sidewalks, and soforth. Other manner of shape stabilization can be included, such ascasting, molding, pressing, or 3D printing of the cementitious slurryusing a pressing, molding, or forming device. The structural componentsare conveyed into a carbonation reactor 112 (e.g., including acarbonation chamber) operated at a temperature in a range of about 50°C. to about 70° C. to react with CO₂ sourced from a flue gas in a(water) condensing atmosphere at sub-boiling conditions. Specifically,during exposure to CO₂, portlandite within a structural component isconverted into limestone (or calcium carbonate (CaCO₃)) by CO₂mineralization. Such mineralized CaCO₃ can provide desirable mechanicalproperties and durability, as well as cementation by forming limestonearound and between aggregates to bind the aggregates to one another.This stage forms a final concrete product as a mineralized,pre-fabricated upcycled concrete product. Fly ash also can serve as acalcium source, and upon slight dissolution or leaching fly ash surfacescan be activated at a relatively high pH (e.g., in portlandite-richenvironments) to provide cohesion/cementation.

In some embodiments, the integration into a primary (exhaust) loop of acoal-fired power plant is achieved with two sub-systems: (I) a wasteheat recycling sub-system, and (II) a two-stage carbonation sub-system.

(I) Waste Heat Recycling

Referring to FIG. 3, the flue gas of a coal-fired power plant typicallyfeatures an outlet temperature between about 120° C. and about 180° C.Thermal energy in the hot flue gas leaving a boiler is typicallyrecovered by an economizer followed by an air pre-heater (APH). Flue gasheat recovery in the APH is performed until the flue gas temperaturedrops to about 150° C. (depending on the type of coal consumed) tomitigate against condensation of sulfuric acid (H₂SO₄) on a surface ofthe APH and downstream ducts or other sub-systems. Cooling of the fluegas below an acid dew point (e.g., about 140° C.) can lead to acidcondensation and deposition which in turn can cause corrosion, fouling,and plugging of the APH, the downstream ducts, and an electrostaticprecipitator (ESP). Such fouling and plugging can result in increasingpressure drop and power consumption to force the flue gas through theAPH. Finally, the flue gas leaving the ESP at about 150° C. to about170° C. can be injected with activated carbon to remove mercury (Hg)traces before entering a flue gas desulfurization scrubber (FGD). TheFGD can be a “wet” system composed of a spray tower in which the fluegas contacts a mist of droplets of an aqueous slurry of sorbentparticles, such as hydrated lime or portlandite (Ca(OH)₂) and limestone(CaCO₃). Water evaporation reduces the flue gas temperature to about 50°C. to about 70° C. at which the desulfurization process is mostefficient. The sorbent particles react with SO₂ in the flue gas to forminsoluble calcium sulfite (CaSO₃), which reacts with oxygen to producegypsum (CaSO₄.2H₂O). In such manner, about 95% of the SO₂ is removedfrom the flue gas stream.

To ensure energy efficient leaching, precipitation and carbonation, theupcycled concrete manufacturing process taps or sources the flue gasline at about 150° C. before the FGD to operate the leaching andprecipitation reactors at about 20° C. to about 90° C. or about 70° C.to about 90° C. (depending on ambient weather and desired leachingrates) and re-injects colder flue gas back into the FGD, albeit abovethe dew point (e.g., >about 140° C. and up to, for example, about 160°C.). The integration points are illustrated in FIG. 3. A set offinned-tube heat exchangers (FTHX, see FIG. 1) that transfer residualheat from the flue gas to a liquid water feeding the leaching andprecipitation reactors 102 and 106 at an effectiveness of about 0.2 orgreater can be used. A mass flow rate of the flue gas leaving the FTHXcan be adjusted to ensure that the temperature does not fall below theacid dew point (e.g., about 140° C.). Finally, if leaching is done atelevated temperatures, a temperature swing process can include a singlepass crossflow heat exchanger (CFHX, see FIG. 1) to transfer heat fromthe hot ion solution leaving the leaching reactor 102 to a solutionfeeding the leaching reactor 102. These various heat recovery measurescan reduce energy costs of the overall process and of the individualsub-systems. The choice of FTHX of some embodiments is given that a heattransfer coefficient on a flue gas side is small and therefore fins aredesired to increase an effectiveness of liquid/gas heat exchange.However, for liquid/liquid heat exchange, a single pass CFHX issufficient. Other types of heat exchangers also may be included.

(II) Two-Stage Carbonation Cycle

Mineralization of CO₂ (carbonation) is proposed as a viable route forCCU. For example, portlandite (Ca(OH)₂) carbonation is a route toproduce near carbon-neutral building materials. The carbonation involvesthe reaction of CO₂ with portlandite to form limestone (CaCO₃), asdescribed by: Ca(OH)₂+CO₂→CaCO₃+H₂O.

In this reaction, a CO₂-capturing reagent (e.g., portlandite) can be insolution or a slurry form, or in the form of a structural component,while CO₂ can be provisioned as a liquid, a supercritical fluid, or avapor. The reaction can be carried out in a pressurized reactor, sincethe CO₂ partial pressure affects reaction kinetics, althoughnon-pressurized (e.g., ambient pressure) processing is also possible.FIG. 4A displays an extent of a carbonation reaction as a function ofreaction time, for carbonation of portlandite under substantiallyconstant CO₂ partial pressures of about 0.1 MPa, about 1 MPa, and about8 MPa at room temperature. It can be seen that carbonation initiallyproceeds rapidly and achieves more than about 50% completion after about0.5 hours under all three partial pressures. However, the carbonationreaction proceeds more slowly in later stages, and the maximum extent ofreaction that can be achieved becomes markedly dependent on the CO₂partial pressure (see FIG. 4B, which displays the extent of thecarbonation reaction after about 1 hour of reaction time as a functionof CO₂ partial pressure). Therefore, as high a CO₂ pressure aspractical, or as high a CO₂ concentration as practical, would typicallyhave to be provisioned during the later stages of the reaction toachieve a high reaction speed and a desired carbonation level.

The upcycled concrete process can divert the scrubbed flue gas that issecured post-desulfurization, namely after the FGD, into the carbonationreactor 112 (see FIG. 3). The flue gas of a coal-fired power planttypically includes about 12% to about 15% of CO₂ (v/v). To obtain acorresponding carbonation speed as with substantially pure CO₂, a gasmixture can be pressurized to obtain a same CO₂ partial pressure,calculated as a product of an overall gas pressure and a CO₂concentration. The CO₂ concentration in the gas mixture can be enrichedwith technologies such as chemical absorption or membrane separation.The enriched gas mixture may then be pressurized, if specified, toobtain a high CO₂ partial pressure. However, additional challengesarise, since CO₂ is consumed and removed from the gas mixturecontinuously during the reaction. As such, the CO₂ concentration, aswell as the overall gas pressure, reduces as the reaction proceeds in atypical carbonation reaction, leading to diminishing CO₂ partialpressure towards a later stage of reaction. This impacts the late stagereaction speed and restricts a final carbonation level.

To address this issue in a comparative carbonation reaction, a high CO₂partial pressure can be maintained through the reaction by either (i)increasing an overall gas pressure in a later stage, which can incur asubstantial energy cost associated with gas compression, or (ii)enriching a gas mixture with a higher initial CO₂ concentration (or ahigher purity CO₂), wherein the throughput of a CO₂ enrichment processbecomes a constraining factor, and where considerable CO₂ remains in thegas mixture upon completion of carbonation. These options areproblematic for CCU at industrial scale, as CCU relies on achieving ahigh reaction speed (thus a high production throughput) with a lowenergy input.

To resolve this issue, the CO₂ partial pressure condition can bereversed by a two-stage carbonation process. In a first pre-carbonationstage, a carbonation reaction is conducted using a gas with a low CO₂concentration (e.g., pressurized or not), such as a mixture of a fluegas and an exhaust gas recycled at the end of the carbonation reaction.Once the gas becomes CO₂-depleted, the gas is replaced in a second stageby a CO₂-enriched gas (e.g., pressurized or not), such as untreated fluegas or CO₂-enriched flue gas, to finish the second stage of thereaction. The exhaust gas from the second stage is recycled and reusedin the preceding stage to enhance a proportion of CO₂ captured. Thisprocess is flexible, and can incorporate CO₂-enrichment technologies(e.g., membrane separation) and pressurization to further enhance thereaction kinetics and CO₂ capture efficiency. In such cases, optimalprocess conditions for the two-stage carbonation can be determined froma process model.

To obtain optimal process conditions for the two-stage carbonationreaction, a process model can be constructed and process parameters canbe determined as follows.

1) A CO₂ partial pressure during the second carbonation stage is firstselected for maximum reaction extent (e.g., about 1 MPa for portlanditecarbonation as FIG. 4B shows modest improvement is obtained by furtherincreasing the pressure), although the selected pressure also can beaffected by an equipment used (e.g., by a concentration achieved byenrichment together with a pressure allowed by a reactor).

2) An allowed CO₂ pressure (or concentration) drop is also determinedfrom the sensitivity of the reaction extent to the pressure (e.g., theslope in FIG. 4B). From here, a total amount of CO₂ captured during agiven period (determined from a target carbonation level and a quantityof CO₂-capturing reagent, such as portlandite, which will be processedduring the period) is divided into two parts, p₁ and p₂, representingthe CO₂ that will be captured in the first and second stages,respectively. These parameters determine the residual CO₂ in the gasafter the second stage and the carbonation extent at the end of firststage. Since the exhaust gas from the second stage still has arelatively high CO₂ concentration, the exhaust gas will be recycled inthe first stage during the next batch. If additional CO₂ is to be addedto reach p₁, additional gas (e.g., a flue gas) will be mixed with therecycled gas, an amount of which can be calculated from p₁, the CO₂concentration of the recycled gas, and the allowed concentration dropduring carbonation. With these values, the CO₂ concentration specifiedfor the first stage can then be calculated, and a reaction time tocomplete the first stage under a given pressure can be determined fromreaction rate data. Finally, a ratio p₁:p₂ and the pressure during thefirst stage are optimized to reduce the reaction time and enhance CO₂capture under given constraints such as the pressure allowed by thereactor and enrichment process throughput.

FIG. 4C shows a sub-system of some embodiments for the carbonationprocess including a pressurized reaction chamber 400 integrated with aCO₂-enrichment component 402 (e.g., configured to provide about 8.8×enrichment in CO₂ concentration from a starting concentration of about7.7 mol. %) and pressurization up to about 2 MPa via a pair ofmixer/compressors 404 and 406. Comparing to a carbonation processwithout the two-stage carbonation cycle, the depicted sub-system canreduce an energy cost from compression by about 40% if a same level ofCO₂ capture is obtained by compressing gas to compensate for thedecrease in CO₂ partial pressure. The sub-system also uses about 50%less CO₂-enriched gas to obtain an equal amount of CO₂ capture at a samethroughput.

Referring to FIG. 4C, a portion of a flue gas (e.g., having a CO₂concentration of about 7.7 mol. %) is combined with a recycled gas(e.g., having a CO₂ concentration of about 37 mol. %) in themixer/compressor 404 to obtain a pressurized gas mixture, which is thenintroduced into the pressurized reaction chamber 400 to perform a firststage carbonation of a CO₂-capturing reagent, such as in the form of astructural component or another form. Another portion of the flue gas issubjected to enrichment by the CO₂-enrichment component 402 (e.g., toyield an enriched CO₂ concentration of about 68 mol. %), and is combinedwith the recycled gas in the mixer/compressor 406 to obtain apressurized gas mixture, which is then introduced into the pressurizedreaction chamber 400 to perform a second stage carbonation. A partialpressure of CO₂ (or a CO₂ concentration) in the gas mixture asintroduced in the second stage of carbonation is greater than a partialpressure of CO₂ (or a CO₂ concentration) in the gas mixture asintroduced in the first stage of carbonation, and is greater than apartial pressure of CO₂ (or a CO₂ concentration) remaining in the gasmixture at the end of the first stage of carbonation, although overallpressures of the introduced gas mixtures can be substantially the same(e.g., about 2 MPa). A controller 408 (e.g., including a processor 410and an associated memory 412 connected to the processor 410 and storingprocessor-executable instructions) can be included to direct operationof various components of the sub-system shown in FIG. 4C.

It is noted that the two carbonation stages can be performed in the samepressurized reaction chamber 400 by replacing a gas phase reactant usinga gas exchange mechanism (e.g., including a pump 414 and themixer/compressors 404 and 406, along with valve(s), duct(s), and soforth) connected to the pressurized reaction chamber 400, withoutconveying partially carbonated solid or slurry materials from onechamber to another chamber. Additional carbonation stages can beincluded to implement multi-stage processes to further mitigate a dropin CO₂ partial pressure during each carbonation stage. Thus, forexample, a multi-stage carbonation process, in general, can include 2,3, 4, 5, or more carbonation stages, and where additional gas phasereagent including CO₂ is introduced in an i^(th) stage to mitigate adrop in CO₂ partial pressure during a preceding (i−1)^(th) stage.

In another example implementation, a two-stage process can be applied bydirectly utilizing gas-phase CO₂ sources without additionalpressurization or enrichment processes, for example, direct CO₂ capturefrom industrial flue gases. In such cases, the first carbonation stageuses an exhaust gas mixture from a previous carbonation cycle, which hasbeen partially processed by a CO₂-capturing reagent and has a lower CO₂concentration, and as such, a lower CO₂ partial pressure, than anuntreated gas mixture. The partially-treated gas mixture is reacted witha fresh batch of the CO₂-capturing reagent, which is in excess amount sothat a fast reaction kinetics can be obtained. Once the gas is depletedof CO₂, it is replaced with an untreated gas mixture to start the secondcarbonation stage. In this stage, the CO₂-capturing reagent is consumedand the gas becomes partially treated, which is used in the nextcarbonation cycle. By maintaining one of the reagents in excess (e.g.,the CO₂-capturing reagent during the first stage and CO₂ in the gasmixture during the second stage, respectively), the process achievessteady reaction kinetics and high material utilization efficiency.Process optimizations can be carried out similarly as discussed before.As an example, flue gases emitted from power plants typically have CO₂concentrations from about 4 mol. % to about 20 mol. %, which translatesto CO₂ partial pressures from about 0.004 MPa to about 0.02 MPa underambient pressure. Depending on the CO₂-capturing reagent, the secondcarbonation stage can be designed to allow about 30% to about 70% dropin the CO₂ partial pressure, for example, to exhaust when the CO₂partial pressure drops to about 0.0012 MPa to about 0.014 MPa, while thefirst carbonation stage can be designed to scrub CO₂ from the exhaustedgas to desired concentration, for example, about 400 ppm to about 1000ppm.

Example

The following example describes specific aspects of some embodiments ofthis disclosure to illustrate and provide a description for those ofordinary skill in the art. The example should not be construed aslimiting this disclosure, as the example merely provides specificmethodology useful in understanding and practicing some embodiments ofthis disclosure.

Clinkering-Free Cementation by Fly Ash Carbonation

Overview:

The production of ordinary portland cement (OPC) is a CO₂ intensiveprocess. Specifically, OPC clinkering reactions involve substantialenergy in the form of heat, and also result in the release of CO₂ fromboth the de-carbonation of limestone and the combustion of fuel toprovide heat. To create alternatives to this CO₂ intensive process, thisexample demonstrates a route for clinkering-free cementation bycarbonation of fly ash, which is a by-product of coal combustion. It isshown that in moist environments and at sub-boiling temperatures,Ca-rich fly ashes react readily with gas-phase CO₂ to produce robustlycemented solids. After seven days of exposure to vapor-phase CO₂ atabout 75° C., such formulations achieve a compressive strength of about35 MPa and take up about 9% CO₂ (by mass of fly ash solids). On theother hand, Ca-poor fly ashes due to their reduced alkalinity (lowabundance of mobile Ca- or Mg-species), show reduced potential for CO₂uptake and strength gain—although this deficiency can be somewhataddressed by the provision of supplemental or extrinsic Ca-containingreagents. The roles of CO₂ concentration and processing temperature arediscussed, and linked to the progress of reactions and the developmentof microstructure. The outcomes create pathways for achievingclinkering-free cementation while providing the beneficial utilization(“upcycling”) of emitted CO₂ and fly ash, which are two abundant, butunderutilized industrial by-products.

Introduction:

Over the last century, for reasons of its low-cost and the widespreadgeographical abundance of its raw materials, OPC-concrete has been usedas the primary material for the construction of buildings and otherinfrastructure. However, the production of OPC is a highly energy—andCO₂—intensive process. For example, at a production level of about 4.2billion tons annually (corresponding to >about 30 billion tons ofconcrete produced), OPC production accounts for about 3% of primaryenergy use and results in about 9% of anthropogenic CO₂ emissions,globally. Such CO₂ release is attributed to factors including: (i) thecombustion of fuel involved for clinkering the raw materials (limestoneand clay) at about 1450° C., and (ii) the release of CO₂ during thecalcination of limestone in the cement kiln. As a result, about 0.9 tonsof CO₂ are emitted per ton of OPC produced. Therefore, there is greatdemand to reduce the CO₂ footprint of cement, and secure alternativesolutions for cementation for building and infrastructure construction.

Furthermore, there exist challenges associated with the production ofelectricity using coal (or natural gas) as the fuel source. For example,coal power is associated with significant CO₂ emissions (about 30% ofanthropogenic CO₂ emissions worldwide), and also results in theaccumulation of significant quantities of solid wastes such as fly ash(about 600 million tons annually worldwide). While OPC in the binderfraction of concrete can be replaced by supplementary cementitiousmaterials (SCMs) such as fly ash, the extent of such utilization remainsconstrained. For example, in the United States, about 45% of fly ashproduced annually is beneficially utilized to replace OPC in theconcrete. In spite of supportive frameworks, such constrained use is dueto factors including: (i) the presence of impurities includingair-pollution control (APC) residues and unburnt carbon as a result ofwhich some fly ashes are non-compliant (e.g., as per ASTM C618) for usein traditional OPC concrete, due to durability concerns, and, (ii)increasing cement replacement (fly ash dosage) levels to greater thanabout 25 wt. % is often associated with extended setting times and slowstrength gain resulting in reduced constructability of the concrete.

Accordingly, there is a demand to valorize or beneficially utilize(“upcycle”) vapor and solid wastes associated with coal powerproduction. However, given the tremendous scale of waste production,there is a demand to secure upcycling opportunities of some prominence;for example, within the construction sector wherein large-scaleutilization of upcycled materials can be achieved. This condition can besatisfied if the “upcycled solution” is able to serve as an alternativeto OPC (and OPC-concrete) so long as it is able to fulfill thefunctional and performance specifications of construction. Mineralcarbonation (conversion of vapor phase CO₂ into a carbonaceous mineral,such as CaCO₃) is proposed as a route to sequester CO₂ in alkalineminerals. In such a process, CO₂ is sequestered by the chemical reactionof CO₂ streams with light-metal oxides to form thermodynamically stablecarbonates; thus allowing permanent and safe storage of CO₂. Whiledifferent alkaline waste streams can be examined to render cementationsolutions, the low production throughput, or severe operating conditions(high temperature and elevated CO₂ pressure) can render comparativesolutions difficult to implement at a practical scale. Therefore, tosynergize the utilization of two abundant by-products from coal-firedpower plants (fly ash and CO₂ in flue gas), this example demonstratesclinkering-free cementation via fly ash carbonation. It is shown thatCa-rich fly ashes react readily with CO₂ under moist conditions, atatmospheric pressure and at sub-boiling temperatures. The influences ofCa availability in the fly ash, CO₂ concentration, and processingtemperature on reaction kinetics and strength gain are discussed. Takentogether, this example demonstrates routes for simultaneous valorizationof solid wastes and CO₂, in an integrated process.

Materials and Methods:

Materials

Class C (Ca-rich) and Class F (Ca-poor) fly ashes compliant with ASTMC618 were used. An ASTM C150 compliant Type I/II ordinary portlandcement (OPC) was used as a cementation reference. The bulk oxidecompositions of the fly ashes and OPC as determined by X-rayfluorescence (XRF) are shown in Table 1. The crystalline compositions ofthe Ca-rich and Ca-poor fly ashes as determined using X-ray diffraction(XRD) are shown in Table 2. It should be noted that these two fly asheswere used since they represent typical Ca-rich and Ca-poor variants inthe United States, and since Ca content can strongly influence theextent of CO₂ uptake and strength development of carbonated fly ashformulations.

TABLE 1 The oxide composition of fly ashes and OPC as determined usingX-ray fluorescence (XRF). Mass (%) Simple Oxide Ca-rich Fly Ash Ca-poorFly Ash Type I/II OPC SiO₂ 35.44 53.97 20.57 Al₂O₃ 17.40 20.45 5.19Fe₂O₃ 7.15 5.62 3.44 SO₃ 2.34 0.52 2.63 CaO 26.45 12.71 65.99 Na₂O 1.900.57 0.17 MgO 5.73 2.84 1.37 K₂O 0.53 1.11 0.31 P₂O₅ 0.95 0.30 0.08 TiO₂1.19 1.29 0.26 Density (kg/m³) 2760 2470 3150 Specific 4292.6 616.8442.6 surface area (SSA, m²/kg)¹ ¹The surface area of the Ca-rich (ClassC) fly ash is overestimated by N₂ adsorption due to the presence ofunburnt carbon. However, based on kinetic analysis of reaction rates inOPC + fly ash + water systems, it can be inferred that the reactivesurface areas of both the Ca-rich and Ca-poor fly ashes are similar toeach other, and that of OPC.

TABLE 2 The mineralogical composition of fly ashes and OPC as determinedusing quantitative X-ray diffraction (XRD) and Rietveld refinement. Mass(%) Ca-rich Composition Fly Ash Ca-poor Fly Ash Type I/II OPC Lime (CaO)1.16 — 0.5 Periclase (MgO) 3.81 0.30 — Quartz (SiO₂) 10.06 16.64 —Calcite (CaCO₃) 0 0 4.60 Mullite (Al₆Si₂O₁₃) 0.86 5.08 — Anhydrite(CaSO₄) 2.80 0.97 1.2 Gypsum — — 1.1 (CaSO₄•2H₂O) Magnetite (Fe₃O₄) 1.661.76 — Merwinite 6.98 — — (Ca₃Mg(SiO₄)₂) Gehlenite (Ca₂Al₂SiO₇) 4.45 — —Ca₂SiO₄ (C₂S) 4.93 — 18.00 Ca₄Al₂Fe₂O₁₀(C₄AF) — — 11.40 Ca₃Al₂O₆ (C₃A)8.03 — 6.30 Ca₃SiO₅ (C₃S) — — 56.50 Amorphous 55.26 75.25 —

Experimental Methods

Particle Size Distribution and Specific Surface Area

The particle size distribution (PSD) of OPC was measured using staticlight scattering (SLS) using a Beckman Coulter LS13-320 particle sizingapparatus fitted with an about 750 nm light source. The solid wasdispersed into primary particles via ultrasonication in isopropanol(IPA), which was also used as the carrier fluid. The complex refractiveindex of OPC was taken as 1.70+0.10i. The uncertainty in the PSD wasabout 6% based on six replicate measurements. From the PSD, the specificsurface area (SSA, units of m²/kg) of OPC was calculated by factoring inits density of about 3150 kg/m³, whereas the SSAs of the fly ashes weredetermined by N₂-BET measurements.

Carbonation Processing

Fly ash particulates were mixed with deionized (DI) water in a planetarymixer to prepare dense suspensions—pastes having w/s=about 0.20 (w/s,water-to-solids ratio, mass basis) which provided sufficient fluiditysuch that they can be poured—following ASTM C192. The pastes were castinto molds to prepare cubic specimens with a dimension of about 50 mm oneach side. Following about 2 hours of curing in the molds at atemperature T=45±0.2° C. and relative humidity RH=50±1%, the specimenswere demolded after which on account of evaporation they featured areduced water content, with w/s=about 0.15, but were able to hold form;that is, they were shape stabilized. At this time, the cubes were placedin a carbonation reactor, a schematic of which is shown in FIG. 5.

Gas-phase CO₂ at atmospheric pressure with a purity of about 99.5%(“pure CO₂”) was used for carbonation. On the other hand, about 99% pureN₂ at atmospheric pressure was used as a control vapor that simulatedambient air (with a CO₂ abundance of about 400 ppm). In addition, asimulated flue gas was created by mixing the pure N₂ and pure CO₂streams to yield a vapor with about 12% CO₂ (v/v) as confirmed using anInficon F0818 gas chromatography (GC) instrument. Prior to contactingthe samples, all vapor streams were bubbled into an open, water-filledcontainer to produce a condensing environment in the reactor (as shownin FIG. 5). Each of the vapors was contacted with the cubical samples attemperatures of 45±0.2° C., 60±0.2° C., and 75±0.2° C.

Compressive Strength

The compressive strengths of the fly ash cubes (both control samples,and those exposed to CO₂) were measured at about 1 day intervalsfollowing ASTM C109 for up to about 10 days. All strength data reportedin this example are the average of three replicate specimens cast fromthe same mixing batch. For comparison, the compressive strengths of neatOPC pastes prepared at w/s=about 0.30, about 0.40, about 0.50, and about0.60 were measured after about 1, about 3, about 7, and about 28 days ofimmersion and curing in a Ca(OH)₂-saturated solution (“limewater”) at25±0.2° C.

CO₂ Uptake by Fly Ash Formulations

CO₂ uptake due to carbonation of the fly ashes was quantified by twomethods: (i) a mass-gain method, and (ii) thermogravimetric analysis(TGA). The mass-gain method was used to estimate the average CO₂ uptakeof the bulk cubic specimen from the mass gain of three replicate cubesfollowing CO₂ contact as given by Equation (1):

$\begin{matrix}{w = \frac{m_{t} - m_{i}}{m_{a}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where, w (g/g) is the CO₂ uptake of a given cube, m_(t) (g) is the massof the specimen following CO₂ contact over a period of time t (days),m_(i) (g) is the initial mass of the specimen, and ma (g) is the mass ofdry fly ash contained in the specimen (estimated from the mixtureproportions). It should be noted that carbonation is an exothermicreaction; thus it can result in the evaporation of water from thesample. However, since curing was carried out in a near-condensingatmosphere, mass measurements before and after carbonation revealed nonoticeable mass loss due to (moisture) evaporation. The ratio of CO₂uptake at time t to that assessed at the end of the experiment (CO₂uptake fraction, a) is given by Equation (2):

$\begin{matrix}{\alpha = \frac{m_{t} - m_{i}}{m_{f} - m_{i}}} & {{Eq}.\mspace{14mu}(2)}\end{matrix}$where m_(f) (g) is the final mass of a given cubical specimen followingabout 10 days of CO₂ exposure.

TGA was used to determine the extent of CO₂ uptake at different depthsin the fly ash cubes, from the surface to the center in about 5 mmincrements. To accomplish so, cubes were sectioned longitudinally usinga hand saw. Then, samples were taken from the newly exposed surfacealong a mid-line using a drill at a sampling resolution of about ±1 mm.The dust and debris obtained during drilling, at defined locations alongthe center-line, were collected and pulverized for thermal analysis in aPerkinElmer STA 6000 simultaneous thermal analyzer (TGA/DTG/DTA)provided with a Pyris data acquisition interface. Herein, about 30 mg ofthe powdered sample that passed an about 53 μm sieve was heated underultra-high purity (UHP)-N₂ gas purged at a flow rate of about 20 mL/minand heating rate of about 10° C./min in pure aluminum oxide cruciblesover a temperature range of about 35-to-about 980° C. The mass loss (TG)and differential weight loss (DTG) patterns acquired were used toquantify the CO₂ uptake by assessing the mass loss associated withcalcium carbonate decomposition in the temperature range about 550°C.≤T≤about 900° C. The mass-based method of assessing the extent ofcarbonation and the spatially resolved TGA method indicate, on average,similar levels of carbonation, as noted below.

X-Ray Diffraction (XRD)

To qualitatively examine the effects of carbonation, the mineralogicalcompositions of fly ash mixtures before and after CO₂ exposure wereassessed using XRD. Here, entire fly ash cubes were crushed and groundinto fine powders, and XRD patterns were collected by scanning fromabout 5-to-about 70° (2θ) using a Bruker-D8 Advance diffractometer in aθ-θ configuration with Cu—Kα radiation (λ=about 1.54 Å) fitted with aVANTEC-1 detector. Representative powder samples were examined to obtainaveraged data over the whole cube. The diffractometer was run incontinuous mode with an integrated step scan of about 0.021° (2θ). Afixed divergence slit of about 1.00° was used during X-ray dataacquisition. To reduce artifacts resulting from preferred orientationand to acquire statistically relevant data, the (powder) sample surfacewas slightly textured and a rotating sample stage was used.

Scanning Electron Microscopy (SEM)

The morphology and microstructure of the un-carbonated and carbonatedfly ash mixtures were examined using a field emission scanning electronmicroscope provisioned with an energy dispersive X-ray spectroscopydetector (SEM-EDS; FEI NanoSEM 230). First, hardened samples weresectioned using a hand saw. Then, these freshly exposed sections weretaped onto a conductive carbon adhesive and then gold-coated tofacilitate electron conduction and reduce charge accumulation on the(otherwise) non-conducting surfaces. Secondary electron (SE) images wereacquired at an accelerating voltage of about 10 kV and a beam current ofabout 80 pA.

Thermodynamic Simulations of Phase Equilibria and CO₂ Uptake

To better understand the effects of carbonation on the mineralogy andmechanical property development of carbonated fly ashes, thermodynamiccalculations were carried out using GEM-Selektor, version 2.3 (GEMS).GEMS is a broad-purpose geochemical modeling code which uses Gibbsenergy minimization criteria to compute equilibrium phase assemblagesand ionic speciation in a complex chemical system from its total bulkelemental composition. Chemical interactions involving solid phases,solid solutions, and aqueous electrolyte(s) are consideredsimultaneously. The thermodynamic properties of all the solid and theaqueous species were sourced from the GEMS-PSI database, with additionaldata for the cement hydrates sourced from elsewhere. The Truesdell-Jonesmodification of the extended Debye-Hückel equation (see Eq. 3) was usedto account for the effects of solution non-ideality:

$\begin{matrix}{{\log\;\gamma_{j}} = {\frac{{- {Az}_{j}^{2}}\sqrt{I}}{1 + {B\;\alpha_{j}\sqrt{I}}} + {bI} + {\log_{10}\frac{x_{jw}}{X_{w}}}}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$where γ_(j) is the activity coefficient of j^(th) ion (unitless); z_(j)is the charge of j^(th) ion, α_(j) is the ion-size parameter (effectivehydrated diameter of j^(th) ion, A), A (kg^(1/2)·mol^(−1/2)) and B(kg^(1/2)·mol^(−1/2)·m⁻¹) are pressure, p- and T-dependent Debye-Hückelelectrostatic parameters, b is a semi-empirical parameter that describesshort-range interactions between charged aqueous species in anelectrolyte, I is the molal ionic strength of the solution (mol/kg),x_(jw) is the molar quantity of water, and X_(w) is the total molaramount of the aqueous phase. It should be noted that this solution phasemodel is suitable for I≤2.0 mol/kg beyond which, its accuracy isreduced. In the simulations, Ca-rich and Ca-poor fly ashes were reactedwith water in the presence of a vapor phase composed of: (a) air (about400 ppm CO₂), (b) about 12% CO₂ (about 88% N₂, v/v), and, (c) about 100%CO₂ (v/v). The calculations were carried out at T=75° C. and p=1 bar.The solid phase balance was calculated as a function of degree ofreaction of the fly ash, until either the pore solution is exhausted(constraints on water availability) or the fly ash is fully reacted.

Results and Discussion:

Carbonation Strengthening

FIG. 6A shows the compressive strength development as a function of timefor Class C (Ca-rich) and Class F (Ca-poor) fly ash pastes carbonated inpure CO₂ at about 75° C. The Ca-rich fly ash formulations show rapidstrength gain following exposure to CO₂, particularly during the first 6days. For example, after about 3 days of CO₂ exposure, the carbonatedformulation achieves a strength of about 25 MPa, whereas a strength onthe order of about 35 MPa is produced after about 7 days of CO₂exposure. On the other hand, as also seen in FIG. 6A, when the Ca-richformulation was exposed to N₂ at the same T, RH, and gas flow rate(serving as a “control” system), a strength of about 15 MPa developsafter 7 days, due to limited reaction of a small quantity of readilysoluble Ca-compounds with any available silica, water, and ambient CO₂.As such, the level of strength developed in the control system is lessthan half of that in the carbonated (Ca-rich) fly ash formulation. Theextent of strength development that is noted in the carbonated system issignificant as it indicates that carbonated binders can fulfillcode-based (strength) criteria relevant to structural construction(≥about 30 MPa as per ACI 318).

To provide a point of reference, the compressive strengths of neat-OPCformulations were measured across a range of w/s. For example, FIG. 6Bshows that the compressive strength of a Ca-rich fly ash formulationfollowing exposure to CO₂ for about 7 days at about 75° C. —about 35MPa—corresponds to that of an OPC formulation prepared at w/s of about0.50 and cured in limewater at about 23° C. over the same time period.It is noted, however, that the fly ash formulations show a reduced rateof strength gain after about 7 days—likely due to the consumption ofreadily available species (Ca, Mg) that can form carbonate compounds. Onthe other hand, OPC systems show a strength increase on the order ofabout 30% from about 7 days to about 28 days (a common aging period thatis noted in building codes) of maturation across all w/s.

Furthermore, FIG. 6A also indicates that, unlike the “carbonationstrengthening” seen in Ca-rich fly ash formulations, Ca-poor fly ashsystems showed a strength of ≤about 7 MPa even after about 10 days ofcarbonation, a gain of ≤about 2 MPa following CO₂ exposure vis-à-vis asystem cured in a N₂ atmosphere. This indicates that, in general,Ca-poor fly ashes feature reduced potential for CO₂ mineralization orstrength gain following CO₂ exposure because the [Ca, Mg] availabletherein is either insufficient or not easily available for reaction(e.g., see FIG. 8). This indicates that carbonation strengthening isdominantly on account of the presence of reactive, alkaline compounds,namely Ca- and Mg-bearing compounds (e.g., CaO, MgO, and so forth), andCa present in the fly ash glass (see Tables 1-2), that can react withCO₂. It should also be noted that Ca-rich fly ashes contain cementitiousphases such as Ca₂SiO₄, Ca₂Al₂SiO₇, and Ca₃Al₂O₆ (see Table 2), whichupon hydration (and carbonation) form cementitious compounds such as thecalcium-silicate-hydrates (C—S—H), or in a CO₂ enriched atmosphere,calcite and hydrous silica (e.g., see FIGS. 7-8). As a result, when suchCa-rich fly ash reacts with CO₂ in a moist, super-ambient (butsub-boiling) environment, carbonate compounds such as calcite (CaCO₃)and magnesite (MgCO₃) are formed as shown in FIGS. 7-8. This is notobserved in the Ca-poor fly ash due to both its much lower total [Ca+Mg]content and their lower reactivity (e.g., see FIGS. 7-8, which showslittle if any formation of carbonate minerals following CO₂ exposure).It should be noted that while the extents of reaction of the fly ashes(Ca-rich or Ca-poor) were not explicitly assessed, it is expected thattheir degree of reaction is ≤about 25% for the short reaction times andover the temperature conditions of relevance to this example.

In general, upon contact with water, the reactive crystalline compounds(e.g., CaO, Ca₃Al₂O₆, and so forth) present in a Ca-rich fly ash areexpected to rapidly dissolve in the first few minutes. As the pHsystematically increases, with continuing dissolution, alkaline speciesincluding Na, K, and Ca can be released progressively from the glassycompounds. This can result in the development of a silica-rich rim onthe surfaces of fly ash particles. Pending the presence of sufficientsolubilized Ca, and in the presence of dissolved CO₂, calcite can formrapidly on the surfaces of leached (and other) particles, therebyhelping proximate particles to adhere to each other as the mechanism ofcarbonation strengthening (e.g., see FIGS. 7-9). This is additionallyhelped by the liberation of Ca and Si from the anhydrous fly ash, whosereaction with water results in the formation of hydrated calciumsilicates (see FIGS. 7-8), calcite, and hydrous silica. This issignificant as the hydrated calcium silicates and calcite can feature amutual affinity for attachment and growth.

With extended exposure to CO₂, the hydrated calcium silicates decomposeto form calcite and hydrous silica (as shown in FIG. 7), which can alsooffer cementation. The systematic formation of mineral carbonates inthis fashion induces: (i) cementation, for example, in a manneranalogous to that observed in mollusks, and sea-shells, that bindsproximate particles to each other via a carbonate network, or carbonateformation which ensures the cementation of sandstones, and (ii) anincrease in the total volume of solids formed which results in adensification of microstructure, while ensuring CO₂ uptake (e.g., seeFIG. 7 for scenarios wherein reaction with CO₂ results in an increase insolid volume).

Coming back to ascertaining the ability of flue gas from coal-firedpower plants, as is, to carbonate fly ash, the Ca-rich fly ash wascarbonated in an about 12% CO₂ atmosphere (v/v) at about 75° C. As notedin FIG. 6C, FIG. 7B, and FIG. 7E, CO₂ present in flue gas at relevantconcentrations can readily carbonate fly ash and ensure strength gain,albeit at a slightly reduced rate vis-à-vis pure CO₂ exposure. Thisreduced rate of strength gain (and carbonation) is on account of thelower abundance of dissolved CO₂ in the vapor phase, and hence in theliquid water following Henry's law. However, it should be noted thatafter about 10 days of exposure to simulated flue gas, the strength ofthe Ca-rich fly ash formulation corresponded to those cured in apure-CO₂ atmosphere (FIG. 6C). This is significant, as it demonstrates apathway for clinkering-free cementation by synergistic use of both flyash and untreated flue gas of dilute CO₂ concentrations sourced fromcoal-fired power plants.

To better assess the potential for valorization of diverse industrialwaste streams of CO₂, the effects of reaction temperature on carbonationand strength gain were further examined. As an example, flue gas emittedfrom coal-fired power plants features an exit temperature (TE) on theorder of about 50° C.≤TE≤about 140° C. to reduce fouling and corrosion,and to provide a buoyant force to assist in the evacuation of flue gasthrough a stack. Since heat secured from the flue gas is the primarysource of thermal activation of reactions, the carbonation of Ca-richfly ash formulations and their rate of strength gain were examinedacross a range of temperatures as shown in FIG. 6C. The rate of strengthgain increases with temperature. This is on account of two factors: (i)elevated temperatures facilitate the dissolution of the fly ash solids,and the leaching of the fly ash glass, and (ii) elevated temperaturesfavor the drying of the fly ash formulation, thereby easing thetransport of CO₂ into the pore structure which facilitates carbonation.It should however be noted that the solubility of CO₂ in water decreasesrapidly at temperatures in excess of about 60° C. While in a closedsystem this may suppress the rate of carbonation, the continuous supplyof CO₂ provisioned herein, in a condensing atmosphere ensures thatlittle or no retardation in carbonation kinetics is observed despite anincrease in temperature. It should also be noted that carbonationreactions are exothermic. Therefore, increasing the reaction temperatureis expected to retard reaction kinetics (following Le Chatelier'sprinciple); unless heat were to be carried away from the carbonatingmaterial. Of course, such exothermic heat release can further decreasethe solubility of CO₂ in water by enhancing the local temperature in thevicinity of the reaction zone. As such, several processes including thedissolution of the fly ash solids, leaching of the fly ash glass, andthe transport of solubilized CO₂ through the vapor phase and waterpresent in the pore structure influence the rate of fly ash carbonation.

To more precisely isolate the role of Ca content of the fly ash, furtherexperiments were carried out wherein Ca(OH)₂ or Ca(NO₃)₂ were added tothe Ca-poor fly ash in order to produce bulk Ca contents correspondingto the Ca-rich fly ash. Here, it should be noted that while Ca(OH)₂ wasadded as a solid that was homogenized with the fly ash, Ca(NO₃)₂ wassolubilized in the mixing water. The results shown in FIG. 6D highlightthat although the Ca(OH)₂- and Ca(NO₃)₂-enriched Ca-poor fly ashesexperienced substantial strength increases (about 35%) followingcarbonation as compared to the pristine Ca-poor fly ash, the strengthswere lower than that of the Ca-rich fly ash (see FIG. 6A). Nevertheless,the enhancement in strength observed in the Ca-poor formulations ispostulated to be on the account of both: (a) the pozzolanic reactionbetween the added Ca source and silica liberated from the fly ashresulting in the formation of calcium silicate hydrates (C—S—H), and,(b) the formation of calcite and (hydrous) silica gel by thecarbonation-decomposition of C—S—H, and by direct reaction ofsolubilized Ca with aqueous carbonate species. The carbonation of C—S—Hcan result in the release of free water and the formation of a silicagel with reduced water content, as is also predicted by simulations (seeFIG. 7). However, such water release (and increase in the porosity) doesnot appear to be the cause of the reduced strengths obtained in theCa-poor fly ash formulations. Rather, it appears as though the presenceof reactive Ca intrinsic to the fly ash (glass), and the formation of asilica-rich surface layer to which CaCO₃ can robustly adhere results inhigher strength development in Ca-rich fly ash formations. Given thereduced ability of Ca-poor fly ashes to offer substantialcarbonation-induced strength gain, the remainder of the example focuseson better assessing the effects of CO₂ exposure on Ca-rich fly ashformulations.

Indeed, the electron micrographs shown in FIG. 9 provide additionalinsights into morphology and microstructure development in Ca-rich flyash formulations following exposure to N₂ and CO₂ at about 75° C. forabout 10 days. First, it is noted that the un-carbonated fly ashformulations show a loosely packed microstructure with substantialporosity (FIG. 9A). Close examination of a fly ash particle shows a“smooth” surface (e.g., see FIG. 9B), although alkaline species mighthave been leached from the particle's surface. In contrast, FIGS. 9C-Dreveal the formation of a range of crystals that resemble “blocks andpeanut-like aggregates” on the surfaces of Ca-rich fly ashparticles—post-carbonation. XRD (FIG. 8) and SEM-EDS analyses of thesestructures confirm their composition as that of calcium carbonate(calcite: CaCO₃). The role of calcite and silica gel that form in thesesystems is significant in that such gels serve to reduce the porosity,and effectively bind the otherwise loosely packed fly ash particles(FIG. 9A), thereby ensuring “carbonation strengthening”. Ca-poor fly ashparticles do not show the formation of carbonation products on theirsurfaces, in spite of CO₂ exposure (see FIG. 9E). Furthermore, theaddition of supplemental portlandite to Ca-poor systems results in asomewhat increased level of carbonation product formation on fly ashparticle surfaces (see FIG. 9F). These observations highlight the roleof not just the Ca (and Mg)-content, but also potentially their spatialdistribution on microstructure and strength development in carbonatedfly ash systems.

Carbonation Kinetics

FIG. 10A shows CO₂ uptake by the Ca-rich fly ash formulation asdetermined by thermal analysis (by tracking the decomposition of CaCO₃)as a function of time across a range of curing temperatures. Both therate and extent of CO₂ uptake, at a given time, increase withtemperature. Although the terminal CO₂ uptake (which is a function ofchemical composition) might be proposed to be similar across allconditions, this was not observed over the course of theseexperiments—likely due to kinetic constraints on dissolution, and thesubsequent carbonation of the fly ash solids. Nevertheless, a linearcorrelation between compressive strength evolution and the CO₂ uptake ofa given mixture is noted (see FIG. 10B)—for both Ca-rich and Ca-poor flyash formulations. Significantly, a strength gain on the order of about3.2 MPa per unit mass of fly ash carbonated is realized. It should benoted that the Ca-rich fly ash composition examined herein—in theory—hasthe potential to take-up about 27.1 wt. % CO₂ assuming that all the CaOand MgO therein would carbonate (e.g., see XRF composition in Table 1).Based on the correlation noted in FIG. 10B, realizing the highestmaximum carbonation level—at thermodynamic equilibrium—would produce aterminal strength on the order of about 86 MPa independent of theprevailing reaction conditions (CO₂ concentration, and temperature). Itshould be noted however that achieving this terminal level of CO₂ uptakemay be difficult to achieve in practice due to the time-dependent: (i)formation of carbonate films of increasing thickness which hindersaccess to the reactants, and (ii) formation of a dense microstructurethat hinders the transport of CO₂ through the liquid phase to reactivesites.

Broadly, mineral carbonation (the formation of calcite and/or magnesite)typically takes the form of irreversible heterogeneous solid-liquid-gasreactions. In the case of Ca-rich fly ashes, it includes the processesof dissolution and hydration of the Ca-rich compounds includingβ-Ca₂SiO₄, Ca-rich glasses, CaO, Mg(OH)₂, Ca(OH)₂, and so forth, and thesubsequent precipitation of CaCO₃ and MgCO₃ from aqueous solution, withreference to, for example, Table 2, FIG. 7, and the following reactions:CO₂(g)+H₂O(l)↔H₂CO₃(aq)↔H⁺(aq)+HCO⁻ ₃(aq)  Eq. (4)HCO⁺ ₃(aq)↔H⁺(aq)+CO²⁻ ₃(aq)  Eq. (5)XO(s)+H₂O(l)→X(OH)₂(s)→X²⁺(aq)+2OH⁻(aq), where X═Ca, Mg  Eq. (6)X₂SiO₄(s)+4H⁺(aq)→2X²⁺(aq)+SiO₂(s)+2H₂O(l)  Eq. (7)X²⁺(aq)+CO²⁻ ₃(aq)→XCO₃(s)  Eq. (8)

Simultaneous to the dissolution and hydration of the solids, vapor phaseCO₂ will dissolve in water, as dictated by its equilibrium solubility(as described by Henry's law) at the relevant pH and temperature. Asionized species from the reactants and dissolved CO₂ accumulate in theliquid phase, up to achieving supersaturation—described by the ratio ofthe ion activity product to the solubility product for a given compound,such as calcite—precipitation occurs thereby reducing thesupersaturation level. Ca- or Mg-bearing compounds in the fly ash wouldcontinue to dissolve as the solution remains under-saturated withrespect to these phases due to the precipitation of carbonates, ensuringcalcite and/or magnesite formation until the readily available quantityof these reactant compounds is exhausted and the system reachesequilibrium. It should be noted that in fly ash mixtures, wherein theabundance of alkaline compounds is substantial, where a largeCa/alkaline-buffer exists, the dissolution of gas-phase CO₂ which wouldotherwise acidify the pore solution has little impact on altering thesolution pH.

It should furthermore be noted that, in the case of the fly ash cubestested for compressive strength (following ASTM C109) (see FIG. 10C, andassociated thin-section analysis) or in the case of fly ash particulates(e.g., see FIG. 9), in general, carbonation reactions proceed inwardfrom the surface to the interior and the surface reacts faster than thebulk. The kinetics of such reactions can be analyzed by assessing howthe rate of conversion of the reactants is affected by processvariables. For example, as noted above in FIG. 10A, it is seen thatcarbonation occurs rapidly at short reaction times, and its rateprogressively decreases with increasing reaction time. This nature ofrapid early-reaction, followed by an asymptotic reduction in thereaction rate at later times can be attributed to: (i) the nucleationand growth of carbonate crystals which occurs at early reaction times,and whose rate of formation is a function of the surface area of thereactant, and (ii) a diffusion- (transport-) limited process whichinvolves transport of CO₂ species to microstructure hindered siteswherein carbonation occurs. Such kinetics can be described by ageneralized reaction-diffusion model as shown in the below:

$\begin{matrix}{\left\lbrack {1 - \left( {1 - \alpha} \right)^{\frac{1}{3}n}} \right\rbrack = {kt}} & {{Eq}.\mspace{14mu}(9)}\end{matrix}$where α is the CO₂ uptake ratio (g of CO₂ uptake per g of reactant, herefly ash), t is the time (days, d), k (d⁻¹) is the apparent reaction rateconstant, and n is an index related to the rate-determining step. Forexample, n=1 represents the “contracting volume model” for rapid initialnucleation and growth of products from the reactants from an outersurface of a spherical shape. When n=2, Equation (9) reduces to Jander'smodel for diffusion-controlled reactions, wherein the reaction rate isdetermined by the transport of reactants through the product layer tothe reaction interface. It should be noted that herein, the presence ofliquid water serves to catalyze carbonation reactions, by offering ahigh pH medium that can host mobile CO₃ ²⁻ ions.

FIG. 11 shows fits of Equation (9) to the experimental carbonation datataken from FIG. 10A for different carbonation temperatures. A clearchange in slope is noted just prior to a reaction interval of about 2days. Across all temperatures, initially the slopes (m, unitless) of allthe curves, wherein m=1/n, are on the order of: m=1±0.2, while afterabout 2 days, m=0.5±0.1. The slight deviation of the slopes from theirideal values (n=1 and 2) is postulated to be on account of the wide-sizedistributions of the fly ash particles and the irregular coverage ofparticles by the carbonation products, for example as shown in FIG. 9.The rate constants obtained from the fittings shown in FIG. 10A wereused to calculate the apparent activation energy of the two steps ofcarbonation reactions, namely a topochemical reaction step, followed bya diffusion-limited step. This analysis reveals: (i) E_(a,1)=about 8.9kJ/mole for surface nucleation reactions indicative of a smalldependence of reaction rate on temperature, and (ii) E_(a,2)=about 24.1kJ/mole for diffusion-controlled reaction. That the activation energyfor surface nucleation reaction is much lower than that fordiffusion-controlled reaction indicates that the carbonation reaction isdominated by nucleation and growth of carbonation products initially.However, as carbonation reaction progresses, the precipitation ofcarbonation products results in the formation of a barrier layer on thefly ash particles (see FIG. 9)—that binds the particles together andsimultaneously increases the resistance to the transport of CO₂ speciesto carbonation sites. As a result, the transport step assumes ratecontrol in the later stages of carbonation reactions.

CONCLUSIONS

Results set forth in this example demonstrate that exposure toconcentrations of CO₂ in moist environments, at ambient pressure, and atsub-boiling temperatures can produce cemented solids whose propertiesare sufficient for use in structural construction. Indeed, Ca-rich flyash solids, following CO₂ exposure achieve a strength of about 35 MPaafter about 7 days or so, and take-up about 9% CO₂ by mass of reactants.Detailed results from thermodynamic modeling, XRD analyses, and SEMobservations indicate that fly ash carbonation results in the formationof a range of reaction products, namely calcite, hydrous silica, andpotentially some C—S—H which collectively bond proximate particles intoa cemented solid. Careful analysis of kinetic (rate) data using areaction-diffusion model highlights two rate-controlling reaction steps:(a) where the surface area of the reactants, and the nucleation andgrowth of carbonate crystals there upon is dominant at early reactiontimes (E_(a,1)=about 8.9 kJ/mole), and (b) a later-age process whichinvolves the diffusion of CO₂ species through thickening surficialbarriers on reactant sites (E_(a,2)=about 24.1 kJ/mole). It is notedthat due to their reduced content of accessible [Ca, Mg] species,Ca-poor fly ashes feature reduced potential vis-à-vis Ca-rich fly ashesfor CO₂ uptake, and carbonation strengthening. Although the provision ofextrinsic Ca sources to Ca-poor fly ashes can somewhat offset thisreduced content, the observations indicate that not just the totalamount (mass abundance) of [Ca, Mg], but also its reactivity and spatialdistribution contribute toward determining a fly ash solid's suitabilityfor CO₂ uptake and carbonation strengthening. Furthermore, it is notedthat strength gain is linearly related to the extent of carbonation (CO₂uptake). This indicates a way to estimate strength gain if the extent ofcarbonation can be known, or vice-versa. These observations aresignificant in that they demonstrate a route for producing cementedsolids by an innovative clinkering-free, carbonation based pathway.

Implications on Solid and Flue Gas CO₂ Waste Valorization in Coal-FiredPower Plants:

Electricity generation from coal and natural gas combustion results inthe production of substantial quantities of combustion residues and CO₂emissions. For example, in the United States alone, coal combustion (forelectricity production) resulted in the production of nearly about 120million tons of coal-combustion residuals (CCRs), and about 1.2 billiontons of CO₂ emissions in 2016. While some CCRs find use in otherindustries (e.g., FGD gypsum, fly ash, and so forth), the majority ofCCRs continue to be land-filled. For example, in the United States,about 45-55 wt. % of the annual production of fly ash is beneficiallyutilized—for example, to replace cement in the binder fraction intraditional concrete—while the rest is disposed in landfills. Suchunderutilization stems from the presence of impurities in the fly ashincluding unburnt carbon and calcium sulfate that forms due to thesulfation of lime that is injected for air pollution control (APC),compromising the durability of traditional concrete. The materialsexamined herein, namely fly ashes that are cemented by carbonation,should not be affected by the presence of such impurities—as a result, awide range of Ca-rich fly ash sources—including those containingimpurities, and mined from historical reservoirs (“ash ponds”) can beusable for carbonation-based fly ash cementation. Given that fly ashcarbonation can be effected at sub-boiling temperatures using dilute,untreated (flue-gas) CO₂ streams, the outcomes of this example create apathway for the simultaneous utilization of both solid- and vapor-bornewastes created during coal combustion. Such routes for waste, andespecially CO₂ valorization create value-addition pathways that can beachieved without a need for carbon capture (or CO₂ concentrationenhancement). Importantly, the streamlined nature of this carbonationprocess ensures that it well-suited for co-location (“bolt-on,stack-tap” integration) with large point-source CO₂ emission sitesincluding petrochemical facilities, coal/natural gas fired power plants,and cement plants. In each case, emitted flue gas can be used to provideboth waste heat to hasten chemical reactions, and CO₂ to ensuremineralization without imposing additional criteria for emissionscontrol. The proposed approach is significant since—within a lifecycleanalysis (LCA) framework wherein there is no embodied CO₂ impactassociated with reactants such as coal combustion wastes or emitted CO₂,and wherein processing energy (heat) is secured from the flue gasstream—fly ash carbonation, by virtue of active CO₂ uptake, and CO₂avoidance (by diminishing the production and use of OPC) has thepotential to yield CO₂ negative pathways for cementation, and henceconstruction.

As used herein, the singular terms “a,” “an,” and “the” may includeplural referents unless the context clearly dictates otherwise. Thus,for example, reference to an object may include multiple objects unlessthe context clearly dictates otherwise.

As used herein, the term “set” refers to a collection of one or moreobjects. Thus, for example, a set of objects can include a single objector multiple objects. Objects of a set can be the same or different.

As used herein, the terms “connect,” “connected,” and “connection” referto an operational coupling or linking. Connected objects can be directlycoupled to one another or can be indirectly coupled to one another, suchas via one or more other objects.

As used herein, the terms “substantially” and “about” are used todescribe and account for small variations. When used in conjunction withan event or circumstance, the terms can refer to instances in which theevent or circumstance occurs precisely as well as instances in which theevent or circumstance occurs to a close approximation. For example, whenused in conjunction with a numerical value, the terms can refer to arange of variation of less than or equal to ±10% of that numericalvalue, such as less than or equal to ±5%, less than or equal to ±4%,less than or equal to ±3%, less than or equal to ±2%, less than or equalto ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, orless than or equal to ±0.05%. For example, a first numerical value canbe “substantially” or “about” the same as a second numerical value ifthe first numerical value is within a range of variation of less than orequal to ±10% of the second numerical value, such as less than or equalto ±5%, less than or equal to ±4%, less than or equal to ±3%, less thanor equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%,less than or equal to ±0.1%, or less than or equal to ±0.05%.

Additionally, amounts, ratios, and other numerical values are sometimespresented herein in a range format. It is to be understood that suchrange format is used for convenience and brevity and should beunderstood flexibly to include numerical values explicitly specified aslimits of a range, but also to include all individual numerical valuesor sub-ranges encompassed within that range as if each numerical valueand sub-range is explicitly specified. For example, a range of about 1to about 200 should be understood to include the explicitly recitedlimits of about 1 and about 200, but also to include individual valuessuch as about 2, about 3, and about 4, and sub-ranges such as about 10to about 50, about 20 to about 100, and so forth.

While the disclosure has been described with reference to the specificembodiments thereof, it should be understood by those skilled in the artthat various changes may be made and equivalents may be substitutedwithout departing from the true spirit and scope of the disclosure asdefined by the appended claims. In addition, many modifications may bemade to adapt a particular situation, material, composition of matter,method, operation or operations, to the objective, spirit and scope ofthe disclosure. All such modifications are intended to be within thescope of the claims appended hereto. In particular, while certainmethods may have been described with reference to particular operationsperformed in a particular order, it will be understood that theseoperations may be combined, sub-divided, or re-ordered to form anequivalent method without departing from the teachings of thedisclosure. Accordingly, unless specifically indicated herein, the orderand grouping of the operations are not a limitation of the disclosure.

What is claimed is:
 1. A system for manufacturing a concrete product,comprising: a leaching reactor; a precipitation reactor connected to theleaching reactor; a set of heat exchangers thermally connected to theleaching reactor and the precipitation reactor and configured to sourceheat from a flue gas stream and transfer residual heat from the flue gasto liquid water feeding the leaching and precipitation reactors.
 2. Thesystem of claim 1, wherein the set of heat exchangers includes a set offinned-tube heat exchangers.
 3. The system of claim 1, furthercomprising a capacitive concentrator for controlled concentration ofcalcium ions and/or magnesium ions connected between the leachingreactor and the precipitation reactor.
 4. The system of claim 3, whereinthe capacitive concentrator includes a set of electrodes and anelectrical source connected to the set of electrodes.
 5. The system ofclaim 1, further comprising a carbonation reactor that may be connectedto the leaching reactor and the precipitation reactor and configured tosource carbon dioxide from the flue gas stream.
 6. The system of claim5, further comprising a mixer connected between the leaching reactor,the precipitation reactor, and the carbonation reactor.
 7. The system ofclaim 6, further comprising an extruder or a pressing, molding, orforming device connected between the mixer and the carbonation reactor.8. The system of claim 5, wherein the carbonation reactor includes: areaction chamber; and a gas exchange mechanism connected to the reactionchamber and configured to: expose, during an initial time period,contents of the reaction chamber to a first gas reactant having a firstcarbon dioxide concentration; and expose, during a subsequent timeperiod, the contents to a second gas reactant having a second carbondioxide concentration that is greater than the first carbon dioxideconcentration.
 9. A system for manufacturing a concrete product,comprising: a leaching reactor; a precipitation reactor connected to theleaching reactor; a set of heat exchangers thermally connected to theleaching reactor and the precipitation reactor and configured to sourceheat from a flue gas stream; and a capacitive concentrator forcontrolled concentration of calcium ions and/or magnesium ions connectedbetween the leaching reactor and the precipitation reactor.
 10. Thesystem of claim 9, wherein the set of heat exchangers includes a set offinned-tube heat exchangers.
 11. The system of claim 9, wherein thecapacitive concentrator includes a set of electrodes and an electricalsource connected to the set of electrodes.
 12. The system of claim 9,wherein the capacitive concentrator for controlled concentration ofcalcium ions and/or magnesium ions includes a membrane filtrationdevice.
 13. The system of claim 12, wherein the membrane filtrationdevice comprises at least one nanofiltration membrane or reverse osmosismembrane.
 14. The system of claim 9, further comprising a carbonationreactor that may be connected to the leaching reactor and theprecipitation reactor and configured to source carbon dioxide from theflue gas stream.
 15. The system of claim 14, further comprising a mixerconnected between the leaching reactor, the precipitation reactor, andthe carbonation reactor.
 16. The system of claim 15, further comprisingan extruder or a pressing, molding, or forming device connected betweenthe mixer and the carbonation reactor.
 17. The system of claim 14,wherein the carbonation reactor includes: a reaction chamber; and a gasexchange mechanism connected to the reaction chamber and configured to:expose, during an initial time period, contents of the reaction chamberto a first gas reactant having a first carbon dioxide concentration; andexpose, during a subsequent time period, the contents to a second gasreactant having a second carbon dioxide concentration that is greaterthan the first carbon dioxide concentration.
 18. The system of claim 3,wherein the capacitive concentrator for controlled concentration ofcalcium ions and/or magnesium ions includes a membrane filtrationdevice.
 19. The system of claim 18, wherein the membrane filtrationdevice comprises at least one nanofiltration membrane or reverse osmosismembrane.
 20. A method of manufacturing a carbonated concrete productusing the system of claim 1, wherein the method comprises: subjectingthe solids to dissolution in the leaching reactor to yield an solutioncomprising calcium ions and/or magnesium ions; concentrating the calciumand/or magnesium-ion solution; transferring at least a portion of theconcentrated calcium and/or magnesium-ion solution to the precipitationreactor; inducing precipitation of the calcium and/or magnesium-ionsolution in the precipitation reactor to yield portlandite; forming acementitious slurry including the portlandite; shaping the cementitiousslurry into a structural component; placing the structural component ina carbonation reactor; and exposing the structural component to carbondioxide sourced from the flue gas stream, thereby forming the carbonatedconcrete product.